Wavefront coded imaging systems

ABSTRACT

An infrared imaging system for imaging infrared radiation from an object onto a detector includes optics configured for producing, with the infrared radiation, transverse ray intercept curves that are substantially straight, sloped lines. A wavefront coding element is configured such that a modulation transfer function of the optics and wavefront coding element, combined, exhibits reduced variation, over a range of spatial frequencies and caused, at least in part, by thermal variation within the imaging system, as compared to a modulation transfer function of the optics alone, without the wavefront coding element. A post processor is configured for generating an improved modulation transfer function, over the range of spatial frequencies, as compared to the modulation transfer function of the optics and the wavefront coding element.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation of commonly-owned and U.S.patent application Ser. No. 11/192,572, filed 29 Jul. 2005, now U.S.Pat. No. 7,106,510 which is a continuation of U.S. patent applicationSer. No. 10/407,708, filed Apr. 4, 2003, now U.S. Pat. No. 6,940,649,which is a continuation of U.S. patent application Ser. No. 09/747,788,now abandoned, filed Dec. 22, 2000. This patent application is also acontinuation-in-part of commonly-owned and U.S. patent application Ser.No. 09/070,969, filed May 01, 1998, now U.S. Pat. No. 7,218,448 which isa continuation-in-part of U.S. patent application Ser. No. 08/823,894,filed Mar. 17, 1997, now U.S. Pat. No. 5,748,371, issued May 5, 1998,which is a continuation of U.S. patent application Ser. No. 08/384,257,filed Feb. 3, 1995, now abandoned. Each of the above-mentioned patentsand patent applications are incorporated herein by reference. U.S.patent application Ser. No. 09/875,435, filed Jun. 6, 2001, now U.S.Pat. No. 6,525,302, U.S. patent application Ser. No. 09/875,766, filedJun. 6, 2001, and U.S. patent application Ser. No. 09/766,325 filed Jan.19, 2001, now U.S. Pat. No. 6,873,733, issued Mar. 29, 2005 are eachincorporated herein by reference.

BACKGROUND

Traditional optical design is based on the premise that the only majorcomponents of the imaging system are the optics and detector. Thedetector can be analog (e.g. film) or a digital detector (e.g., CCD,CMOS, etc.). Traditional image processing techniques performed on animage are performed after the image is formed. Examples of traditionalimage processing include edge sharpening and color filter array (CFA)color interpolation. Traditional optics are therefore designed to formimages at the detector that are sharp and clear over a range of fieldangles, illumination wavelengths, temperatures, and focus positions.Consequently, a trade off is made between forming good images, whichrequires optical designs that are larger, heavier, and contain moreoptical elements than are desirable, and modifying the design in orderto reduce size, weight, or the number of optical elements, which resultsin loss of image quality.

A need remains in the art for improved optical designs which producegood images with systems that are smaller, lighter, and contain fewerelements than those based on traditional optics.

SUMMARY

Optical design based on Wavefront Coding enables systems such that theycan be smaller, lighter, and contain fewer optical elements than thosebased on traditional optics. Wavefront Coding systems share the task ofimage formation between optics and digital processing. Instead of theimaging system being primarily composed of optics and the detector,Wavefront Coding imaging systems are composed of optics, the detector,and processing of the detected image. The detector can in general beanalog, such as film, or a digital detector. Since processing of thedetected image is an integral part of the total system, the optics ofWavefront Coded imaging systems do not need to form sharp and clearimages at the plane of the detector. It is only the images afterprocessing that need to be sharp and clear.

Wavefront Coding, in general, corrects for known or unknown amounts of“misfocus-like” aberrations. These aberrations include misfocus,spherical aberration, petzval curvature, astigmatism, and chromaticaberration. System sensitivities to environmental parameters such astemperature and pressure induced aberrations, and mechanical focusrelated aberrations from fabrication error, assembly error, drift, wear,etc., are also reduced with Wavefront Coding. Optical designs based onWavefront Coding can reduce the effects of these aberrations and resultin simpler designs that produce good images.

Optical system designs according to the present invention are improvedin that they have the characteristic that the transverse ray interceptcurves are substantially straight lines. Unlike traditional opticaldesigns, the transverse ray intercept curves for wavefront coded systemsneed not have a near zero slope; the slope, which indicates misfocus,may be substantial, because wavefront coding allows the effects due tomisfocus to be removed. In actual systems, the transverse ray interceptcurves should vary mainly in slope over wavelength, field angles,temperature, etc., but need not be exactly straight lines; some rippleis acceptable. With wavefront coding optical surfaces and postprocessing, good images can be produced.

In one embodiment, an infrared imaging system for imaging infraredradiation from an object onto a detector includes optics configured forproducing, with the infrared radiation, transverse ray intercept curvesthat are substantially straight, sloped lines. A wavefront codingelement is configured such that a modulation transfer function of theoptics and wavefront coding element, combined, exhibits reducedvariation, over a range of spatial frequencies and caused, at least inpart, by thermal variation within the imaging system, as compared to amodulation transfer function of the optics alone, without the wavefrontcoding element. A post processor is configured for generating animproved modulation transfer function, over the range of spatialfrequencies, as compared to the modulation transfer function of theoptics and the wavefront coding element.

Another embodiment is an improvement to an imaging system for imaginginfrared radiation from an object onto a detector, the system includingoptics configured for producing, with infrared radiation, transverse rayintercept curves that are substantially straight, sloped lines. Theimprovement includes a wavefront coding element configured for modifyingphase of a wavefront of the infrared radiation such that a modulationtransfer function of the lens and wavefront coding element, combined,exhibits reduced variation, over a range of spatial frequencies andcaused, at least in part, by thermal variation within the imagingsystem, as compared to a modulation transfer function of the opticsalone, without the wavefront coding element. The improvement alsoincludes a post-processor configured for generating an improvedmodulation transfer function, over the range of spatial frequencies, ascompared to the modulation transfer function of a combination of theoptics and the wavefront coding element.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a single-lens miniature imaging system according to thepresent invention.

FIG. 2 illustrates a series of transverse ray intercept curvesillustrating aberrations at various wavelengths for the system of FIG. 1with wavefront coding removed.

FIG. 3 illustrates distortion curves for the system of FIG. 1 withwavefront coding removed.

FIG. 4 illustrates modulation transfer functions (MTF) for the system ofFIG. 1 with wavefront coding removed.

FIG. 5 illustrates MTFs for the system of FIG. 1 with wavefront coding,but without post processing.

FIG. 6 illustrates MTFs for the system of FIG. 1 with wavefront coding,before and after filtering.

FIGS. 7 a and 7 b illustrate sampled point spread functions (PSF) forthe system of FIG. 1 with wavefront coding and after filtering for twoobject distances.

FIG. 8 shows a low cost microscope objective according to the presentinvention.

FIG. 9 illustrates a series of transverse ray intercept curvesillustrating aberrations at various wavelengths for the system of FIG. 8with wavefront coding removed.

FIG. 10 illustrates MTFs for the system of FIG. 8 with wavefront coding,without wavefront coding, and with wavefront coding and filtering.

FIG. 11 shows a passive athermalized IR imaging system according to thepresent invention.

FIG. 12 illustrates a series of transverse ray intercept curvesillustrating aberrations at various wavelengths for the system of FIG.11 without wavefront coding.

FIG. 13 illustrates MTFs for the system of FIG. 11 without wavefrontcoding.

FIG. 14 illustrates MTFs for the system of FIG. 11 with wavefrontcoding, with and without filtering.

FIG. 15 a illustrates transverse ray intercept curves as typicallyimplemented in traditional imaging systems.

FIG. 15 b shows MTFs for the system of FIG. 15 a.

FIG. 16 illustrates an example of a one dimensional separable filter foruse as a post processing element in the present invention.

FIG. 17 illustrates the magnitude of the transfer function of the filterof FIG. 16.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a single-lens miniature imaging system 100 according to thepresent invention. Lens 102 includes wavefront coding element 104 formedon its second surface. Detector 106 is preceded by an infrared (IR)filter 108 and cover glass 110. Post processing filter 112 performsprocessing on the images captured by detector 106.

The examplary single-lens imaging system (singlet) 100 is designed tomeet the following specifications:

-   -   f=2.5 mm    -   F/#=2.6    -   Length<4.5 mm    -   Material: Polymethylmethacrylate (PMMA)    -   FOV=50°    -   Focus: ∞−30 cm    -   pixel size=6 μm    -   Bayer CFA/100% fill factor    -   MTF>40% at 40 lp/mm.

The exemplary singlet 100, without Wavefront Coding element 104, wasdesigned so that the aberrations that are not corrected by the opticalsurfaces, namely petzval curvature and axial chromatic aberration, are atype of misfocus. Specifically, petzval curvature is a type of misfocuswith field angle, and axial chromatic aberration is misfocus withillumination wavelength. The effect of these aberrations couldhypothetically be corrected within small regions of the image plane bychanging the focus position. By adding a Wavefront Coding surface, theresulting MTFs and PSFs will be insensitive to the focus-likeaberrations. However, the MTFs and PSFs will not be the same as an idealin-focus MTF or PSF from a traditional imaging system. Image processingis required to restore the spatial character of the image and produce asharp and clear image.

The form of the Wavefront Coding surface used in this example is

$\begin{matrix}{{{S\left( {x,y} \right)} = {{\sum{a_{i}{{sign}(x)}{\frac{x}{r_{n}}}^{b_{i}}}} + {a_{i}{{sign}(y)}{\frac{y}{r_{n}}}^{b_{i}}}}},{where}} & \left( {{Eq}.\mspace{14mu} 1} \right)\end{matrix}$the sum is over the index i. Sign(x)=−1 for x<0, +1 for x≧0.

The parameter r_(n) is a normalized radius value. This particularWavefront Coding surface is rectangularly separable and allows for fastsoftware processing. Other forms of Wavefront Coding surfaces arenonseparable and are the sum of rectangularly separable forms. Onenon-separable form is defined as:S(r,θ)=Σα_(i) r ^(a) ^(i) cos(b _(i)θ+φ_(i)),  (Eq. 2)where the sum is again over the subscript i.

There are an infinite number of Wavefront Coding surface forms. TheWavefront Coding surface for singlet 100 in this example is placed atthe stop surface (e.g., Wavefront Coding element 104) and has theparameterized equation:

$\begin{matrix}{{S\left( {x,y} \right)} = {{\sum{a_{i}{{sign}(x)}{\frac{x}{r_{n}}}^{b_{i}}}} + {a_{i}{{sign}(y)}{\frac{y}{r_{n}}}^{b_{i}}}}} & \left( {{Eq}.\mspace{14mu} 3} \right)\end{matrix}$

-   -   and the parameter values for i=1, 2, 3 are:        -   a₁=17.4171, b₁=2.9911 a₂=10.8895, b₂=6 a₃=3.8845, b₃=20.1909            r_(n)=0.459.

FIGS. 2-4 illustrate the performance of system 100 with wavefront codingelement 104 removed, in order to illustrate design requirements andperformance. FIG. 5 illustrates the performance of system 100 withwavefront coding element 104 in place, but without post processingfilter 112. FIG. 6 illustrates the performance improvement with postprocessing 112. FIGS. 7 a and 7 b show PSFs for system 100 with bothwavefront coding and post processing.

FIG. 2 illustrates a series of transverse ray intercept curvesillustrating aberrations at various wavelengths, for the system of FIG.1 with wavefront coding element 104 removed for illustrative purposes.Curves are shown for system 100 at half field angles of 0°, 10°, 20° and25° off axis, and for illumination wavelengths of 450 nm, 550 nm, and650 nm. A slope of zero indicates an in-focus condition. Thus on-axisrays are nearly in focus. But, for off axis field angles, the slopes ofthe transverse ray intercept curves increase dramatically.

There are numerous traditional methods of designing lenses. Most methodstry to balance aberrations in order to improve the off-axis imaging atthe expense of on-axis imaging or system simplicity. Traditional designmethodologies do not attempt to make the transverse ray intercept curvesstraight lines. Instead, the traditional goal is to try to minimize thedistance of a substantial portion of the transverse ray intercept curvesfrom the horizontal axis. In most traditional systems the ray interceptcurves are very different from straight lines, but in general lie closerto the horizontal axis than the off-axis curves shown in FIG. 2. Inother words, in traditional systems the variation from a straighthorizontal line is mainly in the straightness of the line, rather thanin its slope.

FIG. 15 a (prior art) illustrates traditional transverse ray plots.These plots are taken from “Practical Computer Aided Lens Design”,Gregory Hallick Smith, William Bell, Inc., Richmond 1998. Note that theplot for near on axis rays do look similar to straight horizontal lines,and thus produce an in focus image. Refer also to FIG. 15 b which showsassociated MTFs for this system. The MTFs for near on axis rays aregood. But as the rays move further off axis, the plots in FIG. 15 aquickly deviate from being straight lines. Their associated MTFs in 15 balso quickly degrade.

The transverse ray intercept curves of FIG. 2 are essentially straightlines, both on and off axis. This is a deliberate design goal, becausethe use of wavefront coding element 104 and post processing filter 112can bring the captured images into focus, so long as the curves withoutwavefront coding are essentially straight lines through the origin, evenif the lines are significantly sloped. The effect of the slope isremoved by adding wavefront coding and post processing.

The aberration petzval curvature gives rise to transverse ray interceptcurves, with slopes that are a function of field angle. Axial chromaticaberration gives rise to ray intercept curves with slopes that are afunction of illumination wavelength. From FIG. 2, both of these featuresare part of the transverse ray intercept curves in this examplarydesign.

FIG. 3 illustrates distortion curves for system 100 of FIG. 1, withwavefront coding element 104 removed. The distortion is less than 0.2%.If distortion was large enough then additional digital processing mightbe required to reposition image points into a non-distorted image. Table1 lists the optical prescription of this lens, again without theWavefront Coding surface. Units are in mm, and the total length is 4.1mm. Aspheric terms describe rotationally symmetric forms of r^(order)with order equal to 4, 6, 8, etc.

TABLE 1 Surface Radius Thickness Material Diameter Obj Inf Inf 0 1 2.0771.7133 PMMA 2 Stop −2.236 0.6498 1.4 3 Inf 1.1 BK7 3.4 4 Inf 0.55 BK73.4 Img 0.1 3.4 Surface Conic 4th Asph. 6th Asph. 8th Asph. Obj 0 1−1.299 −.000375 −.010932 −.00603 Stop −3.140 −.01049 3 0 4 0 Img

FIG. 4 illustrates MTFs for system 100 of FIG. 1, without wavefrontcoding element 104. These MTFs correspond to the transverse rayaberration curves of FIG. 2. The MTFs are for half field angles 0, 15,and 25 degrees with wavelengths of 550 nm. The MTFs include the pixelMTF due to the Bayer color filter array detector with six micron pixelsand 100% fill factor. The on-axis MTF is essentially diffractionlimited. The large drop in MTF off-axis is due to the large amount ofpetzval curvature that is unavoidable in traditional single lens designswith a large field of view. This singlet without wavefront coding 104does not meet the MTF specification of greater than 40% modulation at 40lp/mm for all field angles. But, due to its design for Wavefront Coding,modifying the second surface with a Wavefront Coding surface on element104 will lead to acceptable MTF modulation values when combined withdigital processing. By changing the wavefront coding element 104 eithermore or less sensitivity to misfocus aberrations can be formed.

FIG. 5 illustrates MTFs for system 100 of FIG. 1 with wavefront codingelement 104 in place but without post processing filter 112. The systemis focused at infinity. The half field angles shown are 0, 15, and 25degrees. The wavelength is 550 nm. These MTFs have very little variationwith field angle due to the addition of the Wavefront Coding surface, ascompared to FIG. 4. Pixel MTF due to the Bayer CFA has again beenincluded. The Bayer CFA with 6 μm 100% fill factor pixels has a Nyquistspatial frequency of about 42 lp/mm. Note that there are purposely nozeros in the MTFs below the detector's Nyquist spatial frequency.

FIG. 6 illustrates MTFs for system 100 of FIG. 1, with wavefront codingelement 104 and after processing filter 112. Applying a single digitalfilter in processing block 112 gives the optical/digital MTFs shown inFIG. 6. The MTFs before filtering are as shown in FIG. 5. The MTFs afterprocessing filter 112 at the spatial frequency of 40 lp/mm are all above40% as specified by the design specifications. The level of the MTFsafter processing could further be increased beyond that of thetraditional diffraction-limited case, but possibly at the expense of alower signal to noise ratio of the final image.

FIGS. 7 a and 7 b illustrate sampled two-dimensional PSFs for system 100of FIG. 1, with wavefront coding element 104 and after processing filter112. FIG. 7 a shows the processed PSFs when the object is at infinity.FIG. 7 b shows the processed PSFs when the object is at 30 cm. ThesePSFs are for 550 nm wavelength and half field angles of 0, 15, and 25degrees. After filtering, these PSFs have nearly ideal shapes. Thissinglet 100 when combined with wavefront coding and digital filteringthus easily meets the system specifications.

In one preferred embodiment, processing filter 112 is a rectangularlyseparable digital filter. Rectangularly separable filters are morecomputationally efficient (counting the number of multiplications andadditions) than full 2D kernel filters. Separable filtering consists offiltering each row of the image with the 1D row filter and forming anintermediate image. The columns of the intermediate image are thenfiltered with the 1D column filter to provide the final in-focus image.The separable filter used for this exemplary singlet has the samefilters for rows and columns.

FIG. 16 illustrates an example of a one dimensional separable filter asused as processing filter 112. Coefficients are represented as realvalues, but can be quantified into integer values for fixed pointcomputations. The sum of the filter coefficients equals approximately 1.The coefficients were determined with a least squares algorithm byminimizing the squared difference between the filtered wavefront codedoptical transfer functions (OTF) and a desired MTF with a value greaterthan 40% at 40 lp/mm. The width of the filtered PSFs of FIGS. 7 a and 7b are also minimized with the least squares algorithm. Changes in thefiltered PSFs are minimized in regions away from their central peaks.FIG. 17 illustrates the magnitude of the transfer function of the filterof FIG. 16. The zero spatial frequency value is 1.

Wavefront coding microscope objective 800 is designed to meet thefollowing objectives:

-   -   magnification=10×    -   N.A.=0.15    -   Distortion<1%    -   7 micron square pixels with 100% fill factor    -   VGA grayscale detector    -   Optical material: PMMA.

The depth of field of traditional microscope objectives is described bythe numerical aperture (NA) and the imaging wavelength. The wavefrontcoding objective can have a depth of field that is independent of the NAof the objective. The depth of field can be large enough to introduceprospective distortion to the final images. Regions of the object thatare farther from the objective will appear smaller than regions of theobject closer to the objective. Both near and far regions can imageclearly with a large depth of field. Since the depth of field oftraditional objectives is small, prospective distortion is not commonwith traditional objectives, especially with high NA. Prospectivedistortion can be reduced or eliminated by designing wavefront codingobjectives that are telecentric. In telecentric imaging systems themagnification of the object is independent of the distance to theobject.

FIG. 9 illustrates a series of transverse ray intercept curvesillustrating aberrations at various wavelengths, for system 800 of FIG.8, with wavefront coding element 806 removed. The ray intercept curvesof FIG. 9 describe the performance of the system at wavelengths 450 nm,550 nm and 650 nm for the image field heights of on-axis 0.0 mm, 1.2 mmand 2.8 mm. Full scale is +/−100 microns. Notice that each of these rayintercept curves vary mainly in slope, as required by the presentinvention (e.g., the shape of the curves are essentially the same whenthe slope components of the curves are not considered). While theseplots are not quite as close to perfectly straight lines as those inFIG. 2, they can still be considered to be sloped substantially straightlines.

The major aberration apparent in this design is axial chromaticaberration, with a smaller amount of petzval curvature and lateralchromatic aberration. Without Wavefront Coding this lens would imagepoorly in white light, although it might produce a reasonable image in asingle color. Tables 2 and 3 give the optical prescription for thissystem. Table 3 gives rotationally symmetric aspheric terms for thesystem.

TABLE 2 Radius Surface of curv Thickness Material Diameter Conic Obj Inf2.45906 0.6357861 0 1   1.973107 1.415926 Acrylic 1.2 −1.680295 2−2.882275 0.7648311 1.2 −1.029351 Stop Inf 0.1 Acrylic 0.841 0 4 Inf25.83517 0.841 0 Img 6.173922

TABLE 3 Surface 4th 6th 8th 10th 12th 14th 1 0.013191 −0.22886 0.139609−0.250285 −0.18807 0.193763 2 −0.008797 0.017236 0.007808 −0.2232240.160689 −0.274339 Stop −0.018549 −0.010249 −0.303999 1.369745 11.245778−59.7839958

Wavefront coding element 806 is placed at aperture stop 804, and isgiven by the rectangularly separable form of:

$\begin{matrix}{{{S\left( {x,y} \right)} = {{\sum{a_{i}{{sign}(x)}{\frac{x}{r_{n}}}^{b_{i}}}} + {a_{i}{{sign}(y)}{\frac{y}{r_{n}}}^{b_{i}}}}},} & \left( {{Eq}.\mspace{14mu} 4} \right)\end{matrix}$

-   -   and the parameter values for i=1, 2 are:        -   a₁=1.486852, b₁=3.0 a₂=3.221235, b₂=10.0 r_(n)=0.419.

FIG. 10 illustrates MTFs for system 800 of FIG. 8 with wavefront coding,without wavefront coding, and with both wavefront coding and postprocessing-filtering, for illumination at 450 nm. Image field heightsare 0.0 mm, 1.2 mm and 2.8 mm.

FIG. 11 shows a passive athermalized IR imaging system 1100 according tothe present invention. Lens 1102 is composed of silicon. Lens 1104 iscomposed of germanium. Lens 1106 is composed of silicon. The aperturestop 1108 is at the back surface of lens 1106. Wavefront coding surface1110 is on the back surface of lens 1106 (at aperture stop 1108).Processing block 1112 processes the image.

Design goals are as follows:

-   -   F/2    -   f=100 mm    -   3 deg half field of view    -   Illumination wavelength=10 microns    -   20 micron square pixels, 100% fill factor    -   Silicon & germanium optics    -   Aluminum mounts    -   Temperature range of −20° C. to +70° C.

Combined constraints of low F/#, inexpensive mounting material, and wideoperating temperature make this design very difficult for traditionaloptics. Table 4 gives the optical prescription of system 1100.

TABLE 4 Radius Surface of curv Thickness Material Diameter Conic Obj InfInf 0.6357861 0 1 58.6656 5.707297 Silicon 60 0 2 100.9934 22.39862 57.60 3 447.046 8.000028 Germanium 32.4 0 4 50.88434 17.54754 32.4 0 5455.597 7.999977 Silicon 29.5 0 Stop −115.6064 57.9967 29.5 0 Img6.173922

The Wavefront Coding surface for IR system 100 of this example theparameterized equation:

$\begin{matrix}{{{S\left( {x,y} \right)} = {{\sum{a_{i}{{sign}(x)}{\frac{x}{r_{n}}}^{b_{i}}}} + {a_{i}{{sign}(y)}{\frac{y}{r_{n}}}^{b_{i}}}}},} & \left( {{Eq}.\mspace{14mu} 5} \right)\end{matrix}$

-   -   and the parameter values for i=1, 2 are:        -   a₁=16.196742, b₁=3.172720 a₂=−311.005659, b₂=20.033486            r_(n)=18.314428.

FIG. 12 illustrates a series of transverse ray intercept curvesillustrating aberrations at various wavelengths, for system 1100 of FIG.11, with wavefront coding element 1110 removed. The ray intercept curvesof FIG. 11 describe the performance of system 1100 at a wavelength of 10microns, on axis field points for ambient temperatures of +20° C., −20°C., and +70° C. Full scale is +/−100 microns. Again these plots can beconsidered to be substantially straight lines. While they have more“wiggle” than the plots of FIGS. 2 and 9, in each case, if the plot werefitted to the closest straight line, the wiggles would not stray farfrom the line.

FIG. 13 illustrates on-axis MTF curves for system 1100 without wavefrontcoding at three temperatures (+20°, −20° and +70°). Performance isnearly diffraction limited at +20°, but drops dramatically with changesin temperature.

FIG. 14 illustrates MTFs for system 1100 of FIG. 11, with wavefrontcoding, both with and without filtering by processing block 1112. Theillumination wavelength is 10 microns. The MTFs without filtering aresignificantly different from diffraction limited MTFs, but vary littlewith temperature. Thus, processing block 1112 is able to correct theimages. The MTFs after filtering are near diffraction limited for allthree temperatures (+20°, −20° and +70°). Filtered MTFs extend only tothe Nyquist frequency of the 20 micron detector, or 25 lp/mm.

One preferred way to define what constitutes a transverse ray interceptcurve that is a “substantially straight line” is to look at the MTFsover the entire useful range of the system with wavefront codingapplied. These curves must be very close to each other, in order for thepost processing to be able to move all MTFs to the desired performancelevel. Compare the MTFs of FIG. 4 (e.g., no wavefront coding) to thoseof FIG. 5 (e.g., wavefront coding). The FIG. 5 MTF curves are very closetogether. In FIG. 6, post processing has moved the MTFs to an acceptablelevel. More sophisticated post processing could improve the MTFs muchfurther, to nearly diffraction limited performance, so long as thepreprocessing curves are close enough together. Post processing couldnot accomplish this goal with the curves of FIG. 4, because they are notclose together.

FIG. 10 also illustrates this concept. The MTF curves without wavefrontcoding do not track each other. The curves with wavefront coding areclose together. Thus, the curves with wavefront coding after postprocessing are very good.

In FIGS. 13 and 14, the MTF curves without wavefront coding (e.g., FIG.13) are far apart, but the MTF curves with wavefront coding (e.g., FIG.14) are close enough together that the post processing curves are nearlydiffraction limited.

In FIG. 13, it can be seen that the on-axis MTF (at +20° C., meaningessentially no temperature related misfocus) is essentially diffractionlimited. This is the best case traditional MTF for this system. The MTFsat other temperatures, though, have greatly reduced performance due totemperature related effects.

Now consider the upper set of MTFs of FIG. 14, with wavefront coding andafter processing. The MTFs are nearly identical. Thus the associatedtransverse ray intercept curves can be considered to be substantiallystraight lines, since they are close enough to straight to giveessentially ideal MTFs.

For other systems, a lower level of performance may be acceptable, andconsequently the deviation of the transverse ray intercept curves from astraight line may be larger. Such a situation would result if a fastlens (say F/2) is used with a digital detector, with, for example, 10micron pixels. In 500 nm illumination, the diffraction limited MTF forthe optical system would extend to 1000 lp/mm, but the highest spatialfrequency that could be measured by the detector would be only 50 lp/mm.Thus, aberrations that alter the highest spatial frequencies of theoptics are of no consequence, because they will not be measured by thedetector. Note that while the transverse ray intercept curves may havenoticeable deviations from a straight line (corresponding to the higherspatial frequencies), the transverse ray intercept curves are still“substantially straight lines” according to our definition, because theMTFs with wavefront coding are close together. The MTFs underconsideration are those that correspond to the useful range of theparticular system being considered.

Compare the MTFs of FIGS. 6, 10, and 14 with wavefront coding (e.g.,useful range MTFs for embodiments of the present invention) with theMTFs resulting from traditional design of FIG. 15 b. These traditionalMTFs are quite far apart, so post processing could never give adequateperformance. These curves are generally 50% or more apart, whereas thewavefront coding curves in FIGS. 6, 10, and 14, are within an average of20% of each other over the useful range of the system, and, in the caseof FIG. 10, are within an average of 10% of each other over the usefulrange of the system.

The major aberration apparent in the design of FIG. 11 is temperaturerelated misfocus. Without Wavefront Coding, this lens would image poorlyover a range of temperatures.

1. An infrared imaging system for imaging infrared radiation from anobject to a detector, comprising: optics configured for producing, withthe infrared radiation, transverse ray intercept curves that aresubstantially straight, sloped lines; a wavefront coding elementconfigured for modifying phase of a wavefront of the infrared radiationsuch that a modulation transfer function of the optics and wavefrontcoding element, combined, has reduced variation, over a range of spatialfrequencies and caused, in part, by thermal variation within the imagingsystem, as compared to a modulation transfer function of the opticsalone, without the wavefront coding element; and a post processorconfigured for generating an improved modulation transfer function, overthe range of spatial frequencies, as compared to the modulation transferfunction of the optics and the wavefront coding element.
 2. The infraredimaging system of claim 1, wherein the wavefront coding element isformed on a surface of the optics.
 3. The infrared imaging system ofclaim 1, wherein the system includes an aperture stop, and wherein thewavefront coding element is disposed substantially at the aperture stop.4. The infrared imaging system of claim 1, wherein the optics consist ofa single lens.
 5. The infrared imaging system of claim 4, wherein thelens includes a front surface facing the object and a back surfaceopposite the front surface, and wherein the wavefront coding element isformed on one of the front surface and the back surface.
 6. The infraredimaging system of claim 1, wherein the wavefront coding element isfurther configured such that the modulation transfer function of theoptics and wavefront coding element exhibits the reduced variation overa field of view of at least approximately fifty degrees.
 7. The infraredimaging system of claim 1, wherein the post processor includes a digitalfilter.
 8. The infrared imaging system of claim 7, wherein the digitalfilter includes a separable filter.
 9. The infrared imaging system ofclaim 1, wherein the wavefront coding element forms one of arectangularly separable surface, a non-separable surface, and a surfacehaving a sum of rectangularly separable forms.
 10. In a system forimaging infrared radiation from an object to a detector, the systemincluding optics configured for producing, with the infrared radiation,transverse ray intercept curves that are substantially straight, slopedlines, an improvement comprising: a wavefront coding element configuredfor modifying phase of a wavefront of the infrared radiation such that amodulation transfer function of the optics and wavefront coding element,combined, has reduced variation, over a range of spatial frequencies andcaused, in part, by thermal variation within the system, as compared toa modulation transfer function of the optics alone, without thewavefront coding element; and a post processor configured for generatingan improved modulation transfer function, over the range of spatialfrequencies, as compared to the modulation transfer function of theoptics and the wavefront coding element.
 11. In the system of claim 10,the further improvement wherein the wavefront coding element is formedon a surface of the optics.
 12. In the system of claim 11, the furtherimprovement wherein the wavefront coding element is formed on a surfaceof the optics furthest from the object.
 13. In the system of claim 10,the further improvement wherein the optics consists of a single lens.14. In the system of claim 10, the further improvement wherein thewavefront coding element is further configured such that the modulationtransfer function of the optics and wavefront coding element exhibitsthe reduced variation over a field of view of at least approximatelyfifty degrees.
 15. In the system of claim 10, the further improvementwherein the post processor includes a digital filter.
 16. In the systemof claim 15, the further improvement wherein the digital filter includesa separable filter.
 17. In the system of claim 10, the furtherimprovement wherein the wavefront coding element forms one of arectangularly separable surface, a non-separable surface, and a surfacehaving a sum of rectangularly separable forms.